1. Field of the Invention
The present invention relates to a backscattering type radio frequency identification (RFID) communication system, and more particularly, to a backscattering type RFID communication system in which an RFID tag transmits a signal back to an RE reader by using energy of a carrier wave transmitted by the RFID reader.
2. Description of the Related Art
A communication system in which a radio frequency identification (RFID) tag transmits a signal to an RFID reader by using energy of a carrier wave transmitted by the RFID reader, is referred to as a backscattering communication system.
Referring to FIG. 1, in a backscattering communication system according to the related art, communications between an RFID reader and an RFID tag are performed in such a way that, when the RFID reader transmits a query signal for a predetermined time step, the RFID tag transmits a response signal back for the next time step.
Generally, a frequency band used in RFID communications is designated at a predetermined range in each country, and approval of an RFID-related product is regulated not to deviate from the frequency range.
Thus, when communications between a plurality of RFID readers and a plurality of RFID tags are performed in a restricted frequency band, the frequency band is divided into a plurality of frequency channels, and each RFID reader transmits and receives an RF signal within the range of a designated frequency channel.
As illustrated in FIG. 2, when a query signal of an RFID reader is transmitted upon a particular frequency channel, an RFID tag transmits back a response signal having a frequency that is the same as or in the proximity of a carrier wave by using backscattering. In FIG. 2, the response signal having a slightly lower or higher frequency than a frequency of the carrier wave is transmitted back within the same frequency channel as that of the carrier wave. Such a case is referred to as in-channel backscattering. Besides, there are boundary channel scattering by which a response signal is transmitted back at a frequency corresponding to a boundary between a frequency channel of a carrier wave and a frequency channel adjacent to the frequency channel of the carrier wave and adjacent-channel scattering by which a response signal is transmitted back at a frequency of a frequency channel adjacent to the frequency channel of the carrier wave.
Meanwhile, an RFID communications environment in which the number of RFID readers used in a frequency band allocated to an RFID communication system is greater than the number of frequency channels, is referred to as an overcrowded reader environment.
Under the overcrowded reader environment, there are problems in performing backscattering type RFID communications as described above. Since the number of RFID readers is greater than the number of frequency channels, each RFID reader transmits a query signal upon a randomly-selected frequency channel for each time step. In this case, as illustrated in FIG. 3, an RFID reader 2 may transmit a carrier wave frequently for a time step in which an RFID tag 1 responds to a query signal of an RFID reader 1.
Due to the overcrowded reader environment, frequency channels of carrier waves transmitted by two or more RFID readers may be the same frequently. When the frequency channels of the RFID reader 1 and the RFID reader 2 are the same, a response signal of an RFID tag 1 and a query signal of the RFID reader 2 overlap each other frequently at the same frequency channel, as illustrated in FIG. 4.
In general, an RFID tag in a backscattering type RFID communication system transmits back only a part of energy of a carrier wave as a response signal. Thus, intensities of response signals 2 and 3 are relatively weak compared to an intensity of a query signal 1, as illustrated in FIG. 4. Thus, the RFID reader 1 cannot receive the response signals 2 and 3 of the RFID tag 1 due to the query signal 1 of the RFID reader 2.
In order to solve these problems, transmitting and receiving time steps of all RFID readers may be synchronized with one another. However, the method causes the structure of the RFID reader to be complicated, and manufacturing costs may be increased. In addition, when RFID readers manufactured by different manufacturers are used under the same environment, it is difficult that the RFID readers have compatibility therebetween.